Atoms gain or lose electrons to increase:
a. reactivity
c. stability
b. nucleus size
d. atomic mass

Atoms gain or lose electrons to increase their stability. The answer is letter C. Remember noble gases? They follow the octet rule in which their shells are electrons are filled with eight electrons. So the atoms that do not belong to the noble gas tend to be restless. They want their electrons to be filled to follow the octet rule. That is why they tend to share or gain electrons. Suppose you have a compound of CCl4. You know that chlorine can only share 1 electron because 7 of its electrons are filled. Also, in carbon, it can only share 4 electrons because 4 of it are already filled. That is why carbon needs four chlorine to form CCl4.